The subject matter of this application relates to a method of providing conditional access to encrypted media content.
Audio/video media content (AV content), such as programming viewable on a TV appliance, may be broadcast using analog technology by modulating a radio frequency carrier that is propagated over a cable network. Different broadcast TV channels (commonly associated with channel names, such an NBC, CBS and FOX) are impressed on carriers at different frequencies. A receiver in the subscriber premises responds to a channel selection command by selecting the frequency of a desired channel for screening. The receiver may be integrated in the TV appliance or it may be included in a separate device, such as a set-top box (STB). AV content may also be distributed using digital cable technology. The cable network operator may distribute several services (corresponding to the channels of the analog broadcast television domain) by including content for multiple broadcast TV channels in one or more digital bitstreams that are impressed on the cable network at a network headend. A network operator may also distribute digital AV content over a wireless network (satellite or terrestrial). The network operator may also deliver the broadcast content using internet protocol TV (IPTV).
Moreover, pure audio content (i.e. content without a video component) may be broadcast over a cable network for screening using a suitable receiver. The term “screening” is used in this application to cover both listening to audio content and viewing and listening to AV content.
A content distributor that distributes media content over a cable network may wish to limit access to content distributed over the cable network. Limitations may be imposed at several levels. The content distributor may wish to prevent a subscriber who has terminated service from receiving content over a cable drop that extends to the subscriber premises and remains connected to the cable trunk. The content distributor will generally also wish to prevent a current subscriber from receiving access to a premium channel if the subscriber has elected not to pay the subscription fee for that channel. In addition, the content distributor will want to ensure that pay per view content is available only to subscribers who have in fact paid for access to that content. Access to content can be controlled by use of a conditional access system.
In a cable distribution network that employs conditional access on a channel by channel basis, the content for a channel or program to which access is to be limited may be encrypted as a function of a control word using a symmetric scrambling algorithm before being impressed on the cable distribution network.
The screening equipment uses a local decryption key that allows it to decrypt an ECM received with the content and recover the control word. Having recovered the control word, the screening equipment can decrypt the encrypted content and recover the original content for screening by the subscriber.